This invention relates to a method of forming a plastic article and more particularly to a method of forming a plastic pallet.
Pallets have traditionally been formed of wood. Wood pallets however have many disadvantages. For example they are subject to breakage and thus are not reusable over an extended period of time. Wood pallets also take up a considerable amount of valuable space in the warehouse when they are not in use. In an effort to solve some of the problems associated with wood pallets, plastic pallets have been developed and employed with varying degrees of success. One form of highly successful plastic pallet embodies a twin sheet construction in which an upper plastic sheet is thermoformed over a male mold having downwardly extending leg protrusions, a lower plastic sheet is thermoformed over a female mold having leg cavities complemental to the leg protrusions on the male mold, and the sheets are selectably fused or knitted together with the hollow leg protrusions of the upper sheet nested in the hollow leg cavities of the lower sheet to form a plastic pallet having a generally planar platform structure and a plurality of peripherally spaced hollow fused legs extending downwardly from the platform structure and opening in the upper face of the platform structure.
Whereas this twin sheet pallet has been highly successful and has been widely commercialized, problems occur in the forming of the lower sheet since the draw ratio, defined as the total bottom and side wall area of each leg cavity divided by the area of the leg cavity opening in the upper face of the platform structure, is very high with the result that the plastic material of the lower sheet thins out significantly in the bottom regions of the leg cavities in the female mold as the sheet is thermoformed downwardly into the cavities with the result that the lower portions of the side walls of the leg portions of the lower sheet, as well as the bottom walls of the leg portions, are significantly thinned out relative to the starting thickness of the sheet. Since this is a region of high impact and high wear in the completed pallet, the relatively thin configuration of the lower sheet in the bottom regions of the legs can result in premature pallet failure. In effort to overcome this problem, it has been proposed to utilize a thicker sheet of plastic for the bottom sheet than for the top sheet so that, despite the leg thinning resulting from the high draw ratio in the female mold, the final composite thickness of the lower portions of the legs of the completed pallet will be acceptable. However, this solution results in a heavier pallet for a given strength specification, as well as in a more expensive pallet.